Membrane switches are economical to produce, and have become universally prevalent in keypads for consumer items such as cell phones, handheld game devices. Similar products are generally manufactured with membrane switches. Membrane switches can be formed and used as keypads or any other input device. In general, membrane switches are momentary-on contact switches in which at least one contact is on, or made of, a flexible substrate.
By way of example and not of limitation, the membrane switch comprises a flexible substrate layer having a contact that is shaped like a dome. When force is applied to the dome, the dome deforms and closes the switch. Thus, the membrane switch comprises springing means for each contact point on the keypad.
For a keypad in which the membrane switch is in a resting position, the conductive elements of the switch are out of contact. When the “dome” of the flexible substrate layer is pushed, the “dome” yields and the conductive elements of the dome touches another contact point and forms a detectable circuit. As soon as pressure is released from the dome, the membrane switch returns to its normal “dome” shape and this returns the switch to its normally open position.
Referring to FIG. 1, there is shown a block diagram of a printed circuit board (PCB) used in the cell phone industry. The illustrative PCB 100 has a 4×3 keypad matrix in which the first row corresponds to the 1, 2, and 3 numerical keys of a typical keypad, the second row corresponds to the 4, 5, and 6 keys, and so on. Each button locus on the PCB 100 has a corresponding set of concentric conductive circles. Outer circle 102 represents the outer-most portion of the key location that has conductive material on the PCB. Circle 104 marks the end of the conductive material deposited between circles 102 and 104. Circle 106 has as its interior a single conductive area.
If the conductive areas are blackened as in an actual PCB, two concentric circles, 108 and 110, are visible. For the illustrative membrane switch, a key press electrically connects these two conductive areas 108 and 110, enabling detection of the key press.
The prior art membrane switches provide limited tactile feedback. For example, the key press which connects the conductive areas for FIG. 1 is limited to having a single level of tactile feedback and a single logical on-off function. To help users differentiate which keys are being used, the existing art has relied on two solutions.
The first solution for providing tactile feedback is based on long-standing electric typewriter or adding machine conventions, in which certain keys have raised markings cast into the top of the keys. This raised markings allows an experienced touch-typist to know where his or her fingers are on the input device. This works for persons having good tactile feel that have memorized the keyboard layout. Additionally, the top of each key is typically large enough that a person's finger can not touch two at a time.
The second solution for providing tactile feedback is to use unusual physical key layouts, which are intended to be more user friendly. For example, in U.S. Pat. No. 6,766,023 which was issued to Kiernan, an example of a modified physical key layout with the intent of improving ergonomics is described. The modified physical key layout requires having a user learn the new keyboard layout.